Evolution

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Evolution
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Evolution

• The change in the gene pool of a population from
  generation to generation.
• It is important to remember that individuals do
  not evolve, populations evolve.
• All the genes in a population are called the gene
  pool.
• The ratio of different alleles in that population
  can change over time.
• As the ratio changes, so evolution occurs.

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Evolution

• Modern evolution theories are a mixture of the
  theories of Darwin together with new evidence
  from our knowledge of genetics, fossil records,
  comparative anatomy, physiology and biochemistry.

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Cuvier (1769 1832)
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Cuvier (1769 1832)

• This French anatomist is largely responsible for
  the development of Palaeontology.
• He recorded the succession of fossils in the
  Paris Basin.
• He recognised that the fossils of simpler
  organisms were in the oldest rock strata, and
  that there were many extinctions.
• Cuvier believed that species were fixed.

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Cuvier (1769 1832)

• He thought that the boundries between strata were
  caused by catastrophes like drought, fires and
  floods.
• Cuvier thought that the effects of extinctions
  were probably localised and that after the
  catastrophe passed, new organisms from the
  surrounding areas would repopulate the area.
• Called Catastrophism.

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Lamarck (1774 1829)

• In 1809 Lamarck proposed that organisms could
  gradually bring about changes in themselves to
  suit the environment and that these changes could
  be passed on to their offspring.
• This was known as the Inheritance of acquired
  characteristics

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Lamarck (1774 1829)

• As far as we now know this is not possible as
  changes in somatic cells can not be passed on to
  offspring.

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Darwin (1809 1882)
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Darwin (1809 1882)

• Charles Robert Darwin is known as the Father of
  Evolution
• He thought that there was a unity of life, and
  that all organisms were related through a common
  ancestor.
• He thought similar things could be grouped
  together, he used Linnaeus taxonomy to group his
  organisms together.

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Darwin (1809 1882)

• Darwin showed that species do not remain
  unchanged over time.

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Darwins Reasoning

• All organisms have a high reproductive rate, but
  food supply and other essentials are limited
  leading to a struggle for existence amongst the
  offspring.
• There is genetic variation amongst the offspring
  and some are better adapted for survival.

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Darwins Reasoning

• Organisms with these variations will survive
  longer and reproduce.
• Over time, each progressive generation will be
  better adapted to the environment. Survival of
  the Fittest.
• Eventually, new species will evolve from old
  ones. This is called Natural Selection.

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Sources of Heritable Variation

• The only things that can cause variation are
  mutations and sexual recombination.

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Mutations

• The only source of new alleles is mutation.
• These mutations are rare and random.
• Only a mutation in cell lines which lead to the
  formation of gametes are passed on.
• These are usually harmful and are normally
  carried in the recessive condition.
• Occasionally a mutation is good for adaptation to
  an environment it may be an advantage to
  offspring.

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Mutation

• Over a long period of time these advantageous
  mutations may become established in the
  population.

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Sexual Recombination

• During meiosis the homologous chromosomes line up
  at the equator of the spindle in pairs, with one
  going to each end of the cell.
• In humans there are 223 (8 388 608) possible
  combinations of chromosomes in a gamete.
• Sexual reproduction then mixes these gametes with
  those from another person who has also had his
  chromosomes separated randomly

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Sexual Recombination

• To increase the variation of genes, the swapping
  of pieces of chromosomes in crossing over at
  meiosis leads to even greater differences.

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Agents that Change Gene Frequency

• Natural Selection
• some combinations of alleles are more likely to
  help survival and reproduction thus their
  frequency in the gene pool will steadily increase
  form one generation to the next.

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Agents that Change Gene Frequency

• all populations has a large range of phenotypes,
  which usually fall into a normal distribution
  curve. The selective forces such as predators,
  competition, disease, food, light, water, climate
  etc act on the phenotypes as follows
• Stabilising selection
• Directional Selection
• Disruptive Selection

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Stabilising Selection

• This favours the average over the extremes.

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Directional Selection

• This favours one extreme over the average and
  other extreme.

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Disruptive Selection

• This favours both extremes over the average.

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Agents that Change Gene Frequency

• Genetic Drift
• This occurs in small populations
• It can cause evolution by chance fluctuations in
  the gene pool of a small population.
• It is the change in the gene pool of a small
  population due to chance.

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Genetic Drift
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Founder Effect

• Founder Effect
• this is due to a limited no of ancestors.

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The Founder Effect
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Agents that Change Gene Frequency

• Mutation
• this is the ultimate source of variation.
• Gene Migration
• (immigration and emigration) when immigrants
  arrive form another population with a different
  gene pool, new alleles are introduced.
• NB mutation and immigration increase genetic
  variety, natural selection, emigration and
  genetic drift decrease it.

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